1. Field of the Invention
The present invention relates to a switching regulator, and more specifically, to a soft-start function of a switching regulator.
2. Description of the Related Art
In recent years, battery-driven devices such as portable phones, portable music players, digital cameras, and personal digital assistants (PDAs) have become increasingly widespread. Many of the devices use dry cells as power supply in view of cost or the easiness of securing power supply outside the house. Further, considering a reduction in running cost or growing environmental awareness, some of the devices are required to operate with a single dry cell. In general, the dry cell has a cut-off voltage of about 0.9 V, and hence a switching regulator is used to boost a voltage ranging from approximately 0.9 V to 1.5 V to a voltage of 3 V or 5 V, which is then supplied to the device as power therefor.
However, stable operation of the switching regulator is very difficult to achieve with a voltage as low as 0.9 V to 1.5 V. For that reason, there is employed a technology in which the switching regulator performs a boosting operation with a pulse (several tens of KHz to several hundreds of KHz) supplied from a square-wave oscillator so as to output a relatively high voltage (1.5 V to 2.0 V) and the boosted voltage is then used as power supply to the switching regulator.
FIG. 4 is a circuit diagram illustrating a conventional switching regulator.
The conventional switching regulator includes a switching regulator control circuit 1 and peripheral circuits. A DC voltage source 34 is a power source of the switching regulator control circuit 1 and has a voltage range of from 0.9 V to 1.5 V, assuming a single dry cell as the DC voltage source 34. A square-wave oscillator 18 is an oscillation circuit for outputting a square-wave pulse clk. A voltage of an output terminal VOUT also serves as a power supply voltage to the switching regulator control circuit 1. A voltage detection circuit 17 monitors the voltage of the output terminal VOUT. If the voltage of the output terminal VOUT is lower than a threshold voltage VTH, a detection signal Vpg of the voltage detection circuit 17 is L. When the detection signal Vpg of the voltage detection circuit 17 is L, the square-wave oscillator 18 is in an operating state. A multiplexer circuit 19 outputs the square-wave pulse clk when the detection signal Vpg of the voltage detection circuit 17 is L, and outputs a signal Vpwm of a PWM comparator 16 when the detection signal Vpg of the voltage detection circuit 17 is H. A buffer circuit 20 drives a power transistor 30.
Before the switching regulator control circuit 1 starts a boosting operation, the output terminal VOUT has a voltage determined by subtracting a forward voltage Vf of a diode 32 from a voltage VIN of the DC voltage source 34. The threshold voltage VTH is set to 1.5 V in this example. In other words, if the voltage VIN is 1.5 V or lower, the output voltage of the output terminal VOUT is 1.5 V or lower, and hence the detection signal Vpg of the voltage detection circuit 17 is L. Accordingly, the multiplexer circuit 19 outputs the square-wave pulse clk of the square-wave oscillator 18. The power transistor 30 is driven by the square-wave pulse clk, and the switching regulator starts the boosting operation. This period is referred to as a start-up period T1.
In the start-up period T1, the detection signal Vpg is L and fixes an output VREF_SS of a soft-start circuit 12 to 0 V, and hence the switching regulator control circuit 1 performs the boosting operation with the square-wave pulse clk without negative feedback control.
Through the boosting operation with the square-wave pulse clk, when the output voltage of the output terminal VOUT exceeds the threshold voltage VTH, the detection signal Vpg of the voltage detection circuit 17 becomes H and the square-wave oscillator 18 suspends its operation. The multiplexer circuit 19 outputs the signal Vpwm of the PWM comparator 16.
When the detection signal Vpg of the voltage detection circuit 17 becomes H, the soft-start circuit 12 starts its operation, entering a soft-start period T2.
FIG. 5 is a circuit diagram illustrating an example of the conventional soft-start circuit 12.
The soft-start circuit 12 operates as follows to output the soft-start reference voltage VREF_SS. A constant current source 113 charges a capacitor 107 to gradually increase a voltage of the capacitor 107. The voltage of the capacitor 107 controls a gate of an N type MOS transistor 105. Accordingly, a reference voltage VREF, which is output from a reference voltage source 13, is output as the gradually-increasing soft-start reference voltage VREF_SS from the N type MOS transistor 105.
Referring to the drawings, a problem inherent in the switching regulator having the above-mentioned configuration is described. FIG. 6 is a graph illustrating an operation of the switching regulator of FIG. 4.
Upon a change from the start-up period T1 to the soft-start period T2, the square-wave oscillator 18 suspends its operation while the soft-start circuit 12 starts its operation. The voltage of the output terminal VOUT has been boosted in the start-up period T1, and hence a feedback voltage FB takes a value corresponding thereto. As can be seen from FIG. 6, however, the reference voltage VREF_SS increases gradually from 0 V. On this occasion, an operational amplifier 14 outputs a voltage Verrout to the PWM comparator 16 so as to maintain an equal magnitude relationship between the feedback voltage FB and the reference voltage VREF_SS. Because the feedback voltage FB is higher than the reference voltage VREF_SS, the voltage Verrout output from the operational amplifier 14 is higher than a voltage waveform of a ramp pulse Vramp of a ramp-wave oscillator 15. Therefore, the PWM comparator 16 does not output a switching pulse and hence the switching regulator does not perform the boosting operation. Accordingly, the output voltage of the output terminal VOUT gradually decreases because of the discharge of a load or the like (period TA). The voltage detection circuit 17 has hysteresis in detection voltage and is designed not to clear a detected state even when the voltage of the output terminal VOUT decreases to some extent. However, if a large load is connected, the voltage of the output terminal VOUT decreases beyond the hysteresis, with the result that the voltage detection circuit 17 may clear the detected state. In this case, the operating mode returns to the start-up period T1 again, in which the boosting operation with the square-wave pulse clk is started. If no change occurs in the load, the start-up period T1 and the period TA are repeated.
In order to solve the above-mentioned problem, there is disclosed a switching regulator having a circuit configuration illustrated in FIG. 7 (see Japanese Patent Application Laid-open No. 2004-166428). The square-wave oscillator 18 is controlled by a comparator 21. The comparator 21 has an inverting input terminal to which the feedback voltage FB is input and a non-inverting input terminal to which a soft-start slope voltage V_SS is input. When the feedback voltage FB is lower than the soft-start slope voltage V_SS, the comparator 21 outputs H to allow the square-wave oscillator 18 to operate. A capacitor Css starts to be charged by a constant current source 22 simultaneously with the start-up. Therefore, the slope voltage V_SS increases simultaneously with the start-up. An operational amplifier 14 has two inverting input terminals, and a reference voltage Vref is input to one of those terminals and the slope voltage V_SS is input to another terminal. Those two inverting input terminals are designed such that only one of the terminals to which a lower voltage is input is enabled. Specifically, the slope voltage V_SS is available until the slope voltage V_SS continues increasing to reach to the reference voltage Vref. Then, when the slope voltage V_SS exceeds the reference voltage Vref, the reference voltage Vref becomes available.
Upon the start-up of the switching regulator, the slope voltage V_SS increases gradually. When the slope voltage V_SS exceeds the feedback voltage FB, the square-wave oscillator 18 starts its operation. Then, the switching regulator performs a boosting operation with the square-wave pulse clk. On the other hand, when the slope voltage V_SS becomes lower than the feedback voltage FB, the square-wave oscillator 18 suspends its operation. In other words, a kind of frequency modulation control is performed so that the voltage of the output terminal VOUT increases following the rise of the slope voltage V_SS.
Therefore, when the voltage of the output terminal VOUT exceeds the threshold voltage VTH and the detection signal Vpg of the voltage detection circuit 17 becomes H, the feedback voltage FB and the slope voltage V_SS are close to each other, with the result that a smooth transition from the start-up to normal control is realized without a time lag corresponding to the period TA as illustrated in FIG. 6.
In the switching regulator of FIG. 7, however, the constant current source 22 charges the capacitor Css to generate the slope voltage V_SS, which makes it very difficult to control the slope voltage V_SS with a low power supply voltage. If the power supply voltage supplied to the constant current source 22 falls below 1 V, it is difficult to maintain constant current characteristics thereof, resulting in a significantly reduced charge current for the capacitor Css. The reduction ratio is possibly 1/10 to 1/100 or more as compared to a situation in which the output voltage of the output terminal VOUT is high and a stable operation of the soft-start circuit 12 is ensured. In this case, the slope of the rise of the slope voltage V_SS becomes gentle in proportion to a current reduction rate, and it takes 10 to 100 times more time to generate the slope voltage V_SS. In other words, the switching regulator requires a significantly long start-up time period, which poses a problem that a device equipped with the switching regulator takes a long time from when a power switch is turned on until the device is enabled for actual use.